The bottom line, if you want to become a reseller, you need to become a Microsoft Partner who resells the entire package, not just product keys.
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you bought an MSDN licensed key, which carries up to 10 activations unlike full packaged retail licenses which only carry 1 activation. The person who sold it to you probably sold it to 10 other persons. Somewhere along the way, one of those persons might have installed it on a second system, activated it, because it went past 10 activation threshold, Microsoft detected it that it was being abused and blocked the key from further use.